Hollow glass spheres

ABSTRACT

Hollow glass spheres having average densities of approximately 0.10 grams/cc to approximately 2.0 grams/cc are prepared by heating solid glass particles. The glass spheres consist essentially of the following ingredients in the following amounts stated as weight percentages: SiO 2  (50 to 57%); R 2  0(2 to 15%); B 2  O 3  (0 to 20%); S(0.05 to 1.5%); RO(2 to 25%); RO 2  (other than SiO 2 ) (0 to 5%); R 2  O 3  (other than B 2  O 3 ) (0 to 10%); R 2  O 5  (0 to 5%); and F(0 to 5%). R represents a metal or an element like phosphorous which combines with oxygen in glass. The sizes of the hollow glass spheres are selected to produce a maximum average strength for a desired average density.

This application is a continuation of application Ser. No. 236,042 filedAug. 24, 1988 now U.S. Pat. No. 4,983,550.

FIELD OF THE INVENTION

This invention relates to hollow glass spheres made by heating solidglass particles.

BACKGROUND OF THE INVENTION

Hollow glass spheres are widely used in industry as additives topolymeric compounds, e.g., as modifiers, enhancers, rigidifiers andfillers. Generally, it is desirable that these spheres be strong toavoid being crushed or broken during further processing of the polymericcompound, such as by high pressure spraying, kneading, extrusion orinjection molding. It also is desirable in many cases that these sphereshave densities close to that of the polymeric compound into which theyare introduced in order that they distribute evenly within the compoundupon introduction and mixing. Furthermore, it is desirable that thesespheres be resistant to leaching or other chemical interaction withtheir associated polymeric- compound.

The method of expanding solid glass particles into hollow glass spheresby heating is well known. See, e.g., U.S. Pat. No. 3,365,315. Glass isground to particulate form and then heated to cause the particles tobecome plastic and for gaseous material within the glass to act as ablowing agent to cause the particles to expand. During heating andexpansion, the particles are maintained in a suspended state either bydirecting gas currents under them or allowing them to fall freelythrough a heating zone. Sulfur, or compounds of oxygen and sulfur,serves as the principal blowing agent.

A number of factors affect the density, size, strength, chemicaldurability and yield (the percentage by weight or volume of heatedparticles that become hollow) of hollow glass spheres. These factorsinclude the chemical composition of the glass; the sizes of theparticles fed into the furnace; the temperature and duration of heatingthe particles; and the chemical atmosphere (e.g., oxidizing or reducing)to which the particles are exposed during heating.

There have been problems in attempting to improve the quality and yieldof hollow glass spheres. One reason is that it was believed that thepercentage of silica (SiO₂) in glass used to form hollow glass spheresshould be between 65 and 85 percent by weight and that a weightpercentage of SiO₂ below 60 to 65 percent would drastically reduce theyield of the hollow spheres. Also, the prior art has overlooked thesignificant improvement in strength of hollow glass spheres achievedthrough the optimization of particle size.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved yield of hollow glass spheres from glass particles.

Another object of the invention is to provide hollow glass spheres whichpermit the use of a wider range of materials. Particularly, theinvention has the object of providing improved hollow glass sphereshaving a content of silica between 50 and 57 percent.

It is a further object of this invention to provide hollow glass sphereswhich exhibit high strength and resistance to crushing or breaking.Also, the invention has the object of providing hollow glass sphereshighly resistant to chemical decomposition or leaching.

It is yet another object of the invention to provide hollow glassspheres of selected average density having a range of sizes whichprovide optimum average strength.

It is still a further object of this invention to provide hollow glassspheres having average densities ranging from 0.10 grams per cubiccentimeter to 2.0 grams per cubic centimeter. Also, the invention hasthe object of providing hollow glass spheres having average densitiesgenerally matching the polymeric compounds into which they areintroduced.

It is still another object of the invention to provide polymericcomposites containing hollow glass spheres as aforesaid.

These objects are met by the present invention.

The present invention provides hollow glass spheres which exhibitgreater average strength and resistance to crushing or breaking thancommercial glass spheres of comparable average density. These glassspheres also are highly resistant to chemical decomposition or leachingwhich enables them to be used with a wide range of polymeric compounds.The yield of hollow glass spheres of the present invention also is veryhigh.

The high strength, chemical resistivity and yield of the hollow glassspheres of the present invention is the result of a new composition ofglass. The strength of these spheres also is the result of optimallyselecting the range of sizes of particles comprising the final product.We have found that for a product of hollow glass spheres of a particulardesired average density, there is an optimum range of sizes of particlesmaking up that product which produces the maximum average strength.

The hollow glass spheres of the present invention can be produced withaverage densities ranging from approximately 0.10 grams per cubiccentimeter to approximately 2.0 grams per cubic centimeter (solid glasshas an average density of approximately 2.5 grams per cubic centimeter)In several advantageous embodiments, these spheres comprise glass thatcontains the following ingredients (that may be present within the glassitself or the hollow cavity within the glass sphere) in the followingamounts stated as weight percentages. (R represents a metal or anelement like phosphorous which combines with oxygen in glass.)

    ______________________________________                                        SiO.sub.2           50-57                                                     R.sub.2 O (alkali metal oxides)                                                                    2-15                                                     B.sub.2 O.sub.3      0-20                                                     S                   .05-1.5                                                   RO                   2-25                                                     RO.sub.2 (other than SiO.sub.2)                                                                   0-5                                                       R.sub.2 O.sub.3 (other than B.sub.2 O.sub.3)                                                       0-10                                                     R.sub.2 O.sub.5     0-5                                                       F                   0-5                                                       ______________________________________                                    

Preferably, the glass has a substantially molar balance of severalalkali metal oxides, such as Li₂ O, Na₂ O and K₂ O, which has been foundto improve chemical durability and resistance to leaching.

Glass particles of the present invention are heated to cause them toexpand into hollow spheres. The heat may be applied using conventionalmethods such as a furnace. The furnace preferably provides a quantity ofexcess gas of between approximately 5 and 25 percent above astoichiometric mixture of gas and air to provide a reducing atmospherefor the particles during heating.

The hollow glass spheres of the present invention can be used in a widerange of polymeric compounds, such as paints, coatings, plastisols,organasols, thermoplastic and thermosetting polymers, rigidifiers andspackling compounds. Because of their high strength and durability,there spheres can be incorporated into a wide range of polymers andsubjected to the high pressures of extrusion and injection molding.

The present invention, including further objects, features andadvantages, will be more fully understood from the following detaileddescription.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

A central feature of the present invention is a new composition of glassused to produce hollow glass spheres. The principal ingredient in thisglass is SiO₂ which is present in an amount between 50 and 57 weightpercent.

An alkali metal oxide (R₂ O) is included in the glass in an amount equalto between 2 and 15 weight percent. Preferably, R₂ O comprises Na₂ O, K₂O and Li₂ O in weight percentages preferably ranging from 2.5 to 12.8,3.8 to 10.0 and 1.0 to 3.0, respectively. Advantageously, each of thesealkali metal oxides is included in the glass in a substantially molarbalance for improved chemical durability and resistance to leaching.Such a balance is substantially achieved with a glass containingapproximately one part by eight Li₂ O, to two parts by weight Na₂ O, tothree parts by weight K₂ O.

B₂ O₃ is present in the glass in a weight percentage of between 0 and 20to lower melting temperature and improve chemical durability.Preferably, the glass composition contains between 6 and 15 weightpercent B₂ O₃.

A weight percentage of sulfur (as an individual element or incombination with another element such as oxygen as, e.g., SO₂ or SO₃) ofbetween 0.05 and 1.5 is present in the glass to provide a blowing agent.If the parameters (time, duration and heating atmosphere) of heating theglass particles remain fixed, the density of the hollow glass spheresdecreases as both the percentage of sulfur, or compounds of oxygen andsulfur, in the glass and the size of the feedstock particles increase.By controlling average feedstock size and the percentage of sulfur inthe glass, the glass spheres of the present invention can be producedwith average densities ranging from 0.10 grams per cubic centimeter to2.0 grams per cubic centimeter.

The glass contains RO in a weight percentage of between 2 and 25.Preferably, these oxides are CaO and ZnO and are present in the glass inweight percentages ranging from 5 to 20 and 1.5 to 4.0, respectively.These oxides improve the chemical durability and resistance to leachingof the glass. CaO, moreover, improves the workability and viscosity ofthe glass over a wider range of temperatures, and ZnO helps the glass toretain sulfur during its preparation.

RO₂ (other than SiO₂) can be present in the glass up to 5 weightpercent. Preferably, however, it is omitted and is present only as animpurity.

R₂ O₃ (other than B₂ O₃ can be present in the glass up to 10 weightpercent. Preferably, this oxide is Al₂ O₃ and is present in a weightpercentage of between 2 and 7. The presence of this oxide improveschemical durability and resistance to leaching.

R₂ O₅ can be present in the glass up to 5 weight percent. Preferably,this oxide is P₂ O₅ and is present in an amount ranging from 0.5 to 4weight percent. A small amount of P₂ O₅ is believed to reduce the glass'melting point and improve its workability.

Flourine can be present in the glass in an amount up to 5 weightpercent. Preferably, flourine is present in a weight percentage rangingfrom 1.5 to 2.5. Flourine improves the workability and viscosity of theglass over a wider range of temperatures and is believed also to act asan additional blowing agent during glass expansion.

In certain preferred embodiments, the glass is prepared by introducingglass-forming materials into a crucible in amounts which will result inthe desired glass composition after heating and cooling. One of ordinaryskill in the art could select appropriate amounts of glass-formingmaterials to prepare glass having the composition of this invention.After heating, the molten materials are quenched in water to produce aglass frit which is ground into the particles used to produce the hollowglass spheres.

The particles are heated and caused to expand in apparatus similar tothat described in U.S. Pat. Nos. 2,619,776 and 2,945,326. Preferably,the glass particles are air classified prior to heating using, forexample, an Alpine American, Model 132 MP, air classifier, to provide aselected range of sizes of particles for feeding into the furnace toproduce an optimum range of sizes of particles in the final glass-sphereproduct.

For a product of hollow glass spheres having a particular desiredaverage density, there is an optimum range of sizes of particles makingup that product which produces the maximum average strength. This rangecan be expressed by the following formula: ##EQU1## where 90P is thesize for which 90% of the particles in the glass-sphere product aresmaller (referred to as the 90th percentile size); 10P is the size forwhich only 10% of the particles in the glass-sphere product are smaller(referred to as the 10th percentile size); 50P is the size for which 50%of the particles in the glass-sphere product are smaller (referred to asthe 50th percentile size); and GQ stands for the gradation quotient. Wehave found that in order to maximize the average strength of a productof hollow glass spheres having a selected average density, the sizes ofparticles making up that product should have a GQ of between 0.8 and1.3. We have found that a GQ within this range maximizes the strength ofthe product independently of its chemical composition.

It should be understood that a product of hollow glass spheres caninclude both solid and hollow glass spheres. All the particles heated inthe furnace do not expand, and most hollow glass-sphere products aresold without separating the hollow from the solid spheres.

The invention will be further understood from the following examples.

EXAMPLE 1

Molten glass was quenched in water to produce a glass frit having achemical composition calculated as follows (amounts are indicated asweight percentages): 53.81% SiO₂ ; 4.29% Na₂ O; 6.36% K₂ O; 2.06% Li₂ O;11.74% B₂ O₃ ; 1.37% SO₃ ; 10.64% CaO; 1.96% ZnO; 4.89% ; 0.98% P₂ O₅ ;and 1.90% F₂. The glass was ground to particles and air classified toprovide the following breakdown of sizes 90% smaller than 46.5 microns(90P=46.5 microns); 10% smaller than 15.2 microns (10P=15.2 microns);and 50% smaller than 29.1 microns (50P=29.1 microns). These particleswere fed into a furnace at the rate of 2.5 pounds per hour using 130cubic feet per hour of gas at a gas/air mixture which was approximately12% excess gas (12% above a stoichiometric mixture). The resultantglass-sphere product had an average density of 0.22 grams per cubiccentimeter at a yield of 92.6% by weight hollow glass spheres. Averagedensity was determined by weighing a sample of the glass-sphere productand determining its volume with an air comparison pycnometer.

The breakdown of sizes of this glass-sphere product was as follows:90P=104 microns; 10P=35.8 microns; and 50P=66.1 microns. Accordingly,its gradation quotient (GQ=90P-10P/50P) was 1.03.

The strength of this glass-sphere product was determined by subjecting asample to hydrostatic pressure and determining the percentage of volumeloss. These percentages were as follows for the following hydrostaticpressures: 21.7% at 500 psi; 45.0% at 750 psi; and 58.6% at 1000 psi. Bycomparison, the strongest commercial glass-sphere product of comparableaverage density (0.23 grams per cubic centimeter) lost 54% of its volumeat 750 psi.

The chemical durability and resistance to leaching of this glass-sphereproduct was determined by subjecting a sample to boiling deionized waterfor one hour under reflux and then testing the conductivity, pH, sodiumcontent and potassium content of the water. The results were as follows:conductivity=61 μmho/cm; ph=8; sodium=4.2 mg/liter; potassium=8.8mg/liter. By comparison, a commercial glass-sphere product of comparabledensity subjected to the same test produced the following results:conductivity=200 μmho/cm; ph=8.8; sodium=21.2 mg/liter; andpotassium=11.6 mg/liter.

EXAMPLE 2

A glass frit was prepared with a calculated chemical composition thesame as Example 1. This frit was ground to provide a breakdown ofparticle sizes as follows: 90P=19.8 microns; 10P=3.6 microns; and50P=11.3 microns. These particles were fed into a furnace at the rate of3.4 pounds per hour with the furnace being fed gas at the rate of 120cubic feet per hour at a gas/air mixture which was approximately 9%excess gas. The resultant glass-sphere product had an average density of0.30 grams per cubic centimeter at a yield of 98.4% by weight hollowspheres. This glass-sphere product was comprised of spheres having thefollowing breakdown of sizes: 90P=44.8 microns; 10P=13.2 microns; and50P=29.5 microns. Accordingly, its GP was 1.07.

The percentage volume loss of this product under hydrostatic pressurewas as follows: 2% at 500 psi; 5% at 750 psi; and 11% at 1000 psi. Bycomparison, the strongest commercial product of comparable averagedensity (0.28 grams per cubic centimeter) had a percentage volume lossof 10% at 500 psi, 23% at 750 psi, and 35% at 1000 psi.

EXAMPLE 3

Molten glass was quenched in water to produce a glass frit having achemical composition calculated as follows (amounts are indicated asweight percentages): 54.18% SiO₂ ; 66% Na₂ O; 3.94% K₂ O; 1.28% Li₂ O;11.82% B₂ O₃ ; 0.69% SO₃ ; 5.64% CaO; 1.97% ZnO; 4.93% Al 0.99% P₂ O₅ ;and 1.92% F₂. The frit was ground to provide particles having abreakdown of sizes as follows: 90P=17.1 microns; 10P=6.0 microns; and50P=11.3 microns. These particles were fed into a furnace at the rate of3.7 pounds per hour using gas at the rate of 147 cubic feet per hour ata gas/air mixture which was approximately 22% excess gas. The resultantglass-sphere product had an average density of 0.611 grams per cubiccentimeter at a yield of 92.5% by weight hollow spheres. This productwas comprised of particles whose sizes broke down as follows: 90P=25.7microns; 10P=10.0 microns; and 50P=17.7 microns. Accordingly, its GP was0.89.

A hydrostatic test of this glass-sphere product produced only a 1.7%volume loss at 3000 psi, and only a 25% volume loss at 10,000 psi. Bycomparison, the strongest commercial product of comparable averagedensity (6 grams per cubic centimeter) had a percentage volume loss of50% at 10,000 psi.

These same two products also were introduced into polymers subjected toinjection molding on the same injection molder. The percentage ofspheres breaking during this molding was insignificant for thisglass-sphere product, but substantial for the commercial product.

EXAMPLE 4

A glass frit having the same chemical composition as Example 3 wasground to provide a breakdown of particle sizes as follows: 90P=8.3microns; 10P=1.8 microns; and 50P=4.8 microns. These particles were fedinto a furnace at the rate of 3.4 pounds per hour using gas at the rateof 115 cubic feet per hour at a gas/air mixture which was approximately12% excess gas. The resultant glass-sphere product had an averagedensity of 1.10 grams per cubic centimeter at a yield of 43% by weighthollow glass spheres. This product was comprised of particles whosesizes broke down as follows: 90P=13.7 microns; 10P=4.0 microns; and50P=8.8 microns. Accordingly, its GP was 1.10.

Hydrostatic pressure testing of this product produced only a 13% volumeloss at 10,000 psi and only a 30% volume loss at 20,000 psi.

The chemical durability and resistance to leaching of this product wasdetermined by subjecting a sample to boiling deionized water underreflux for one hour and then testing the water for pH, ccnductivity, andcontent of total alkali, calcium, boron, and silica. For comparison, thesame test was conducted on commercial soda-lime glass and a borosilicateglass. The results are presented in the following table:

    ______________________________________                                                Conduc-                                                                       tivity   Total                                                                (μmho/                                                                              Alkali   Calcium                                                                              Boron  Silica                                pH      cm)      (mg/cc)  (mg/cc)                                                                              (mg/cc)                                                                              (mg/cc)                               ______________________________________                                        Sample                                                                              8.3   52       8.3     7.1    7.0   0.4                                 Soda- 9.4   101      25.7   18.1   trace  5.4                                 Lime                                                                          Glass                                                                         Boro- 8.5   45       3.6    20.4   18.7   1.1                                 silicate                                                                      Glass                                                                         ______________________________________                                    

These results demonstrate that the overall chemical resistance anddurability of the hollow glass-sphere product exceeds that of thesecommercial glasses.

Thus, the present invention meets its objectives in providing a newchemical composition for hollow glass spheres. This composition produceshollow spheres having high strength, yield and chemical durability. Theaverage densities of these spheres can range from 0.10 grams per cubiccentimeter to 2.0 grams per cubic centimeter. The strength of thesehollow glass spheres is further enhanced through an optimum selection ofparticle sizes.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions to exclude any equivalents of thefeatures described or of portions thereof, it being recognized thatvarious modifications are possible within the scope of the invention.

We claim:
 1. Durable high-strength hollow glass spheres made by heatingsolid glass particles, said spheres having an average density or fromapproximately 0.10 grams/cc to aproximately 2.0 grams/cc and consistingessentially of the following ingredients in the following amounts statedas weight percentages:

    ______________________________________                                        SiO.sub.2         50-57                                                       R.sub.2 O          2-15                                                       B.sub.2 O.sub.3    0-20                                                       S                 .05-1.5                                                     RO                 2-25                                                       RO.sub.2 other than SiO.sub.2                                                                   0-5                                                         R.sub.2 O.sub.3 other than B.sub.2 O.sub.3                                                       0-10                                                       R.sub.2 O.sub.5   0-5                                                         F                 0-5                                                         ______________________________________                                    

wherein R represents a metal or other element, having a valence of 1, 2,3, 4, or 5 as dictated by the formulae set forth above, which combineswith oxygen.
 2. Hollow glass spheres as in claim 1 wherein R₂ O in Na₂O, K₂ O or Li₂ O.
 3. Hollow glass spheres as in claim 1 wherein R₂ O isNa₂ O, K₂ O and Li₂ O which are present in weight percentages of 2.5 to12.8, 3.8 to 10.0 and 1.0 to 3.0, respectively.
 4. Hollow glass spheresas in claim 1 wherein R₂ O is Na₂ O, K₂ O and Li₂ O which are present inthe amounts of approximately one part by weight Li₂ O, to two parts byweight Na₂ O, to three parts by weight K₂ O, such that Li₂ O, Na₂ O andK₂ O are included in the glass in a substantially molar balance. 5.Hollow glass spheres as in claim 1 wherein B₂ O₃ is present in a weightpercentage of 6 to
 15. 6. Hollow glass spheres as in claim 1 wherein ROis CaO or ZnO.
 7. Hollow glass spheres as in claim 1 where RO is CaO andZrO which are present in weight percentages of 5 to 20 and 1.5 to 4.0,respectively.
 8. Hollow glass spheres as in claim 1 wherein RO₂ otherthan SiO₂ is substantially absent.
 9. Hollow glass spheres as in claim 1wherein R₂ O₃ other than B₂ O₃ is Al₂ O₃.
 10. Hollow glass spheres as inclaim 9 wherein Al₂ O₃ is present in a weight percentage of 2 to
 7. 11.Hollow glass spheres as in claim 1 wherein R₂ O₅ is P₂ O₅ and is presentin a weight percentage of 0.5 to 4.0.
 12. Hollow glass spheres as inclaim 1 wherein F is present in a weight percentage of 1.5 to 2.5. 13.Hollow glass spheres as in claim 1 wherein B₂ O₃ is present in a weightpercentage of 11.74 to
 20. 14. A product of durable high-strength hollowglass spheres and solid glass spheres made by heating solid glassparticles, said product having an average density of approximately 0.10grams/cc to approximately 2.0 grams/cc and consisting essentially of thefollowing ingredients in the following amounts stated as weightpercentages:

    ______________________________________                                        SiO.sub.2         50-57                                                       R.sub.2 O          2-15                                                       B.sub.2 O.sub.3    0-20                                                       S                 .05-1.5                                                     RO                 2-25                                                       RO.sub.2 other than SiO.sub.2                                                                   0-5                                                         R.sub.2 O.sub.3 other than B.sub.2 O.sub.3                                                       0-10                                                       R.sub.2 O.sub.5   0-5                                                         F                 0-5                                                         ______________________________________                                    

wherein R represents a metal or other element, having a valence of 1, 2,3, 4, or 5 as dictated by the formulae set forth above, which combineswith oxygen.
 15. A product of glass spheres as in claim 14 wherein R₂ Ois Na₂ O, K₂ O or Li₂ O.
 16. A product of glass spheres as in claim 14wherein R₂ O is Na₂ O, K₂ O and Li₂ O which are present in weightpercentages of 2.5 to 12.8, 3.8 to 10.0 and 1.0 to 3.0, respectively.17. A product of glass spheres as in claim 14 wherein R₂ O is Na₂ O, K₂O and Li₂ O which are present in the amounts of approximately one partby weight Li₂ O, to two parts by weight Na₂ O, to three parts by weightK₂ O, such that Li₂ O, Na₂ O and K₂ O are included in the glass in asubstantially molar balance.
 18. A product of glass spheres as in claim14 wherein B₂ O₃ is present in a weight percentage of 6 to
 15. 19. Aproduct of glass spheres as in claim 14 wherein RO is CaO or ZnO.
 20. Aproduct of glass spheres as in claim 14 wherein RO is CaO and ZnO whichare present in weight percentages of 5 to 20 and 1.5 to 4.0,respectively.
 21. A product of glass spheres as in claim 14 wherein RO₂other than SiO₂ is substantially absent.
 22. A product of glass spheresas in claim 14 wherein R₂ O₃ other than B₂ O₃ is Al₂ O₃.
 23. A productof glass spheres as in claim 22 wherein Al₂ O₃ is present in a weightpercentage of 2 to
 7. 24. A product of glass spheres as in claim 14wherein R₂ O₅ is P₂ O₅ and is present in a weight percentage of 0.5 to4.0.
 25. A product of glass spheres as in claim 14 wherein F is presentin a weight percentage of 1.5 to 2.5.
 26. A product of glass spheres asin claim 14 wherein B₂ O₃ is present in a weight percentage of 11.74 to20.
 27. Durable high-strength hollow glass spheres made by heating solidglass particles, said spheres consisting essentially of the followingingredients in the following amounts stated as weight percentages:

    ______________________________________                                               SiO.sub.2                                                                           50-57                                                                   Na.sub.2 O                                                                           2.5-12.8                                                               K.sub.2 O                                                                            3.8-10.0                                                               Li.sub.2 O                                                                          1.0-3.0                                                                 B.sub.2 O.sub.3                                                                      6-15                                                                   S     .05-1.5                                                                 CaO    5-20                                                                   ZnO   1.5-4.0                                                                 Al.sub.2 O.sub.3                                                                    2-7                                                                     P.sub.2 O.sub.5                                                                      .5-4.0                                                                 F     1.5-2.5                                                          ______________________________________                                    

wherein R represents a metal or other element, having a valence of 1, 2,3, 4, or 5 as dictated by the formulae set forth above, which combineswith oxygen.
 28. Hollow glass spheres as in claim 27 wherein Na₂ O, K₂ Oand Li₂ O are present in the amounts of approximately one part by weightLi₂ O, to two parts by weight Na₂ O, to three parts by weight K₂ O, suchthat Li₂ O, Na₂ O and K₂ O are included in the glass in a substantiallymolar balance.
 29. Hollow glass spheres as in claim 27 wherein B₂ O₃ ispresent in a weight percentage of 11.74 to
 15. 30. An article ofmanufacture comprising a polymeric compound and dispersed thereindurable high-strength hollow glass spheres made by heating solid glassparticles, said spheres having an average density of approximately 0.10grams/cc to approximately 2.0 grams/cc and consisting essentially of thefollowing ingredients in the following amounts stated as weightpercentages:

    ______________________________________                                        SiO.sub.2         50-57                                                       R.sub.2 O          2-15                                                       B.sub.2 O.sub.3    0-20                                                       S                 .05-1.5                                                     RO                 2-25                                                       RO.sub.2 other than SiO.sub.2                                                                   0-5                                                         R.sub.2 O.sub.3 other than B.sub.2 O.sub.3                                                       0-10                                                       R.sub.2 O.sub.5   0-5                                                         F                 0-5                                                         ______________________________________                                    

wherein R represents a metal or other element, having a valence of 1, 2,3, 4, or 5 as dictated by the formulae set forth above, which combineswith oxygen.
 31. An article of manufacture as in claim 30 comprising apolymeric compound and dispersed therein durable high-strength hollowglass spheres wherein B₂ O₃ is present in said spheres in a weightpercentage of 11.74 to
 20. 32. An article of manufacture comprising apolymeric compound and dispersed therein the product of durablehigh-strength hollow glass spheres and solid glass spheres made byheating solid glass particles, said product having an average density ofapproximately 0.10 grams/cc to approximately 2.0 grams/cc and consistingessentially of the following ingredients in the following amounts statedas weight percentages:

    ______________________________________                                        SiO.sub.2         50-57                                                       R.sub.2 O          2-15                                                       B.sub.2 O.sub.3    0-20                                                       S                 .05-1.5                                                     RO                 2-25                                                       RO.sub.2 other than SiO.sub.2                                                                   0-5                                                         R.sub.2 O.sub.3 other than B.sub.2 O.sub.3                                                       0-10                                                       R.sub.2 O.sub.5   0-5                                                         ______________________________________                                    

wherein R represents a metal or other element, having a valence of 1, 2,3, 4, or 5 as dictated by the formulae set forth above, which combineswith oxygen.
 33. An article of manufacture as in claim 32 comprising apolymeric compound and dispersed therein the product of durablehigh-strength hollow glass spheres and solid glass spheres wherein B₂ O₃is present in said product in a weight percentage of 11.74 to
 20. 34. Anarticle of manufacture comprising a polymeric compound and durablehigh-strength hollow glass spheres made by heating solid glassparticles, said spheres consisting essentially of the followingingredients in the following amounts stated as weight percentages:

    ______________________________________                                               SiO.sub.2                                                                           50-57                                                                   Na.sub.2 O                                                                           2.5-12.8                                                               K.sub.2 O                                                                            3.8-10.0                                                               Li.sub.2 O                                                                          1.0-3.0                                                                 B.sub.2 O.sub.3                                                                      6-15                                                                   S     .05-1.5                                                                 CaO    5-20                                                                   ZnO   1.5-4.0                                                                 Al.sub.2 O.sub.3                                                                    2-7                                                                     P.sub.2 O.sub.5                                                                      .5-4.0                                                                 F      1.5-2.5.                                                        ______________________________________                                    


35. An article of manufacture as in claim 34 comprising a polymericcompound and durable high-strength hollow glass spheres wherein B₂ O₃ ispresent in said spheres in a weight percentage of 11.74 to 15.